1. Field of the Invention
This invention relates generally to a grid type device for gating a stream of ions. More particularly, this invention relates to a grid of the type described which gates the stream of ions without a residual or leakage ion current flow.
2. Description of the Prior Art
Many applications such as, for example, ion mobility measurements, require gating or turning off the ion flow for analysis or measurement purposes, as the case may be. This is accomplished by disposing a grid in the ion stream and normal thereto, and electrically energizing the grid to suppress the ion current flow. Prior to the present invention, grids for this purpose have been unable to completely suppress the ion flow and consequently there has been a residual flow of ion current through the grid. This residual flow, or leakage, of ion current results in a background current which may particularly interfere with low level current measurements made for the required purposes.
A typical example of a prior art grid which permits the aforenoted residual ion current flow is known in the art as the Bradbury-Nielsen type. This grid includes two sets of substantially parallel, closely spaced interdigitated wires in separate and distinct planes which are separated by a finite distance. A stream of ions is allowed to flow in a tube or cylinder by establishing a steady D.C. potential gradient along the axis thereof. Then, in order to analyze the nature of the ion stream, the ion flow is momentarily interrupted. This is accomplished by placing the grid constructed as aforenoted in the path of the ion flow and applying a strong D.C. potential between the two sets of wires to produce a D.C. field. This results in cutting off the ion current flow which is normal to the plane of the grid. However, in prior art grids, since the two sets of wires are actually in two planes, the D.C. field produced by the grid is not precisely normal to the direction of the ion current flow and, indeed, may actually have a small component in the direction of said current flow. This results in a small residual ion current flow even during the time when the grid is closed or shut off to accomplish the aforementioned gating affect. The present invention provides a unique structural configuration which obviates this disadvantage.